34 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



crystals with the glass and powdered them together to about the fine- 

 ness represented by a 150-mesh sieve and heated them very slowly. 

 In this condition the glass proved to be very unstable and crystallized 

 readily with a rapid release of its latent heat at about 490 . Very 

 slow heating (10 minutes per 1 degree) gave a temperature a few de- 

 grees lower, but such variations as could be applied within the period 

 of a working day did not suffice, under the most favorable conditions, 

 to change this temperature materially. The first evidence of molec- 

 ular mobility in borax glass, shown in the sticking together of the 

 finest particles (sintering), and the first traces of crystallization and 

 release of latent heat, appeared consistently at about 490 to 500 . 

 Still a third phenomenon attracted our attention to this temperature. 

 On every occasion when borax glass was heated rapidly, either pow- 

 dered or in the solid block, a slight but persistent absorption of heat 

 appeared in this same region and continued over some 20 , after which 

 the original rate of heating returned. We were entirely unable to 

 explain an absorption of heat in an amorphous substance under these 

 conditions except by assuming an actual change of state to exist 

 between amorphous glass and its melt, in which case the absorbed 

 heat would reappear somewhere upon the corresponding cooling curve, 

 which it failed to do. We then reasoned that any assumed change 

 in the molecular structure which would account for an absorption of 

 heat would also be likely to cause an interruption in the continuity 

 of the curve of electrical conductivity, and the relative conductivity 

 was determined throughout this region, but no such interruption 

 appeared. 



Finally the matter was abandoned. The evidence did not appear 

 sufficient to establish any discontinuity in the cooling curve of the 

 glass, so long as no crystallization took place. 



When these relations had been clearly established, we turned again 

 to the feldspars. 



It became clear very early in the investigation that only artificially 

 prepared and chemically pure specimens would be adequate for our 

 purpose. Each of the end members of the series, anorthite and albite, 

 as found in nature, is always intermixed with some quantity of the 

 other, while the intermediate members generally contain iron and 

 potash, and all are liable to inclusions. 



There was nothing new in this plan. Fouque and Levy* had 

 demonstrated the possibility of making pure feldspars by chemical 

 synthesis and had studied their optical properties some years ago. We 

 undertook to prepare much larger quantities than they (200 grams) 



* Synthase des Mineraux et des Roches. 



